1. Technical Field
This invention relates generally to drill bits of the type used in drilling through the material comprising a rock formation such as for an oil well or the like. More particularly, this invention is concerned with a fixed cutter bit of the type which, for example, utilizes polycrystalline diamond cutting elements protruding from the face of the bit to cut through the formation material.
2. Background Information
In drilling a borehole in the earth such as for the recovery of oil or for other purposes, many different types of drill bits have been used. The choice of the appropriate type of bit to be used depends upon many factors. One of the most important of these factors to be taken into consideration is the range of hardnesses that will be encountered during transitional drilling that is when drilling through layers of differing formation harnesses.
Different types of bits work more efficiently against different formation harnesses. For example, roller cone bits are efficiently and effectively used in drilling through formation materials that are of medium to hard hardness. The mechanism for drilling with a roller cone bit is primarily a crushing and gouging action in that the inserts of the rotating cones are impacted against the formation material compressing the material beyond its compressive strength and thereby drilling through the formation. For harder materials, the mechanism for drilling changes from crushing to abrasion.
One form of a prior art fixed cutter bit for use in hard material formations is shown in U.S. Pat. No. 2,729,427. This patent teaches the use of natural diamond granules embedded in the matrix of the bit body at its face. Specifically, the granules are arranged in annular ridges which are spaced radially from each other with interposed valleys absent of granules. As described in U.S. Pat. No. 2,729,427, the drilling action resulting from this structure is a combination of abrasion and fracturing of the hard formation material. The diamond granules scour or abrade away concentric grooves while the rock formation adjacent the grooves is fractured and the matrix material around the diamond granules is worn away.
A somewhat similar prior bit also for use in drilling hard formation material is shown in U.S. Pat. No. 3,106,973 disclosing the use of replaceable and adjustable blades mounted on and protruding from the body of the bit. Each of the blades is comprised of radially spaced sections of diamonds or diamond like cutting elements embedded in a matrix. The body of the blade is of a softer material than the diamond impregnated matrix sections and includes arcuate grooves between radially increasingly larger area matrix sections. In service use, the increasing areas of the matrix sections grind at a uniform rate against the formation material while the softer material in the blade body wears away more quickly with the result that ridges are formed in the bottom of the well bore. The ribs are thought to improve the centering stability of the rotating bit and are purposefully kept thin enough for even hard formation material to break down or off by themselves to be washed away with drilling fluid.
The fixed cutter drill bits of the foregoing character are not particularly well suited for use in softer formations because not only do they inherently drill at low penetration rates but their drilling surfaces containing the diamond or diamond like cutting elements may be easily clogged with less brittle formation material. As a result, when drilling from a hard formation material and into a softer formation material the penetration rate may actually drop over that which may be achieved in harder formation materials.
For the drilling of formation materials in the soft to medium range, another type of mechanism for drilling may be employed. An example of a bit which is particularly designed for stabilized drilling is shown in U.S. Pat. No. 4,932,484. The bit disclosed in this patent utilizes radial sets of cutting elements mounted within supports to protrude from the face of the bit. At least one of the sets of cutting elements extends outward a greater distance from the face of the bit than other cutting elements so that during drilling a bit stabilizing annular groove is formed in the formation material by the extended elements. In rotated profile, the cutting elements overlap each other upon progressing radially outward from the rotational axis of the bit so that all of the formation material across the face of the drill bit is cut.
In contrast to the prior art bits shown in U.S. Pat. Nos. 2,729,427 and 3,106,973 wherein the drilling mechanism is disclosed as being by abrasion and fracturing and the like, the drilling elements or cutters disclosed in U.S. Pat. No. 4,932,484 employ a shearing action to drill through the formation material. Specifically, a sharp aggressive cutting edge on each of the cutters is pushed into the bottom of the borehole as the bit is rotated. The actual mechanics of round polycrystalline diamond cutters of this type when used in soft and medium-soft range formations may be described as a shearing action wherein formation material is removed in layers. Thus, with a bit like the one disclosed in U.S. Pat. No. 4,932,484 it may be envisioned that with each revolution of the bit, a layer of formation material having a contour matching that of the cutters is sheared from the bottom of the borehole, depending upon the depth to which the cutting edges of the cutters penetrate the formation for the amount of weight which is applied through the drilling string upon the bit.
Another form of fixed cutter bit which is taught to be usable in medium-soft to medium formations is shown in U.S. Pat. No. 4,602,691. This bit is disclosed as including sharp triangular and blunt circular cutting elements in overlapped protruding profile from the face of the bit. Specifically, the triangular elements protrude 0.005 inch farther from the face of the bit than the circular elements and, thus cut small relief kerfs in the formation. The circular elements follow thereafter and dislodge the formation between the kerfs.
Because the cost of drilling a borehole is a direct function of the length of time it takes to reach the depth desired, it is always desirable to have bits which drill faster and longer and which are usable over a wider range of differing formation material harnesses. Thus, there exists a need for a bit which is suitable for use in transitional drilling so that the drill string need not be pulled when the hardness of the material changes from a relatively soft to a harder material.